The present disclosure relates to an analog-to-digital (AD) converter having improved differential nonlinearity (DNL).
There has in recent years been a demand for the size reduction of multifunctional mobile terminals for easier portability. To meet the demand, it is necessary to further reduce the sizes of an integrated circuit and a sensor device included in a multifunctional terminal. In particular, when an integrated circuit includes an AD converter that converts an output signal of a sensor device into a digital signal, it is necessary to reduce the area of the AD converter, which uses a large area of the integrated circuit.
On the other hand, the size reduction of a sensor device has led to a reduction in the detection sensitivity of the sensor device, i.e., a reduction in output signal level. To compensate for such a sensitivity reduction, the AD converter has been required to have higher accuracy.
Under such circumstances, a technique has been developed that utilizes a combination of different AD converters to extract higher-order bits and lower-order bits (see Japanese Unexamined Patent Publication (Japanese Translation of PCT Application) No. H10-508167, and H. Chen et al., “A 13-bit, Low-Power, Compact ADC Suitable for Sensor Applications,” Proceedings of 2010 International Symposium on Circuits and Systems (ISCAS), pp. 2414-2417, 2010) However, in such a conventional AD converter, the linearity of the output signal with respect to the input signal deteriorates in some cases.
FIGS. 16A, 16B, 16C, and 16D are diagrams showing an output waveform of a conventional lower-order bit AD converter. As shown in these figures, a missing code occurs at a boundary point of the higher-order bits output by the delta-sigma AD converter. A missing code refers to a phenomenon that a portion of a digital code corresponding to an analog input is not output. Specifically, if differential nonlinearity is greater than or equal to ±1 LSB based on the least significant bit (LSB: the lowest-order bit), an output code is missing.
As described above, AD converters are required to be free from a missing code. However, in the conventional art, it is difficult to reduce or prevent the occurrence of a missing code.